Conventional colour photographic silver halide materials are processed by a process which includes a colour development step. In this step silver halide is reduced to metallic silver in the light-exposed areas and the oxidised colour developer formed in this reaction then couples with a colour coupler and forms image dye. The amount of dye produced is proportional to the amount of silver halide reduced to metallic silver.
Redox amplification processes have been described, for example in British Patent Specifications GB-A-1268126, GB-A-1399481, GB-A-1403418 and GB-A-1560572. In such processes colour materials are developed to produce a silver image (which may contain only small amounts of silver) and then treated with a redox amplifying solution to form a dye image. The redox amplifying solution contains a reducing agent, for example a colour developing agent, and an oxidising agent which is more powerful than silver halide and which will oxidise the colour developing agent in the presence of the silver image which acts as a catalyst. Oxidised colour developer reacts with a colour coupler (usually contained in the photographic material) to form image dye. The amount of dye formed depends on the time of treatment or the availability of colour coupler rather than the amount of silver in the image as is the case in conventional colour development processes. Examples of suitable oxidising agents include peroxy compounds including hydrogen peroxide, cobalt (III) complexes including cobalt hexammine complexes, and periodates. Mixtures of such compounds can also be used.
Since the amplifying solution contains both an oxidising agent and a reducing agent it is inherently unstable. That is to say unlike a conventional colour developer solution, amplifier solutions will deteriorate in less than an hour even if left in a sealed container. The best reproducibility for such a process has been obtained by using a "one shot" system, where the oxidant is added to the developer and the solution mixed and used immediately (or after a short built in delay) and then discarded. This leads to the maximum solution usage possible with maximum effluent and maximum chemical costs. As a result the whole system is unattractive especially for a minilab environment where minimum effluent is required. It is believed that it is these shortcomings that have inhibited commercial use of this process.
Japanese Specification 64/44938 appears to describe such a system in which a silver chloride colour material is processed in a low volume of a single-bath amplifier solution. The processes described therein however fall short of what is required in the fully commercial environment for exactly the reasons given above.
Published International Patent Application WO-A-91/12567 (which corresponds to British Patent Application No. 9003282.2) describes a method and apparatus for photographic processing in which a minimum amount of processing solution can be used in a processing tank which is thin and has a low volume. In order to overcome the inherent deterioration problem due to the instability of the processing solutions used, the method and apparatus described result in the need for high recirculation and/or replenishment rates.
One difficulty with processing photographic material continuously in apparatus described in WO-A-91/12567 is that the heat capacity of the processing tank plus its contents is low when compared with normal processing tanks which contain tens of liters of processing solution. When a continuous web of photographic material is passed through the processing tank, a noticeable drop in temperature of the processing solution occurs. This temperature drop will depend on the temperature of the web. An additional drop in temperature is also observed when replenisher is added to the processing tank. In the case of a low volume processing tank, the replenisher forms a larger proportion of the total volume of the tank than normal. For a colour development stage, for example, variation in the processing solution temperature is undesirable and will lead to variable photographic sensitometry.
One solution to the above problem is to ensure that the photographic material and replenisher solutions are at the same temperature as the desired processing solution temperature in the processing tank. However, it may be difficult to do this in relation to the photographic material.
Another solution to the problem is to place the processing tank and its contents in a thermostatically controlled bath to maintain the desired temperature. This will be practicable if the heat transfer through the tank to the processing solution is high enough to give a constant temperature whether photographic material is being transported through the tank or not. However, this solution suffers from the problem of having to control the temperature of a large bath in order to maintain the processing tank and its contents at a constant desired temperature.
It is therefore an object of the present invention to provide apparatus and method which overcome the problems mentioned above.